What hardware design engineers need to know about PCB design issues

Q: When doing high-speed multi-layer PCB design, ResistanceCapacitor The selection of the package of the device, what is the main basis? Commonly used for those packages, can give a few examples.

A: 0402 is commonly used in mobile phones; 0603 is commonly used for high-speed signals; the smaller the package, the smaller the parasitic parameters, of course, the same package of different manufacturers There is a big difference in high frequency performance. It is recommended to use high frequency dedicated components in critical locations.

Q: multi-layer board layout needs What should you watch out for?

A: When laying out the multi-layer board,Because the power supply and the ground layer are in the inner layer, be careful not to have a suspended ground plane or power plane. Also make sure that the vias that hit the ground are connected to the ground plane. Finally, add some test points to some important signals to facilitate measurement when debugging.

Q:through hole and blind hole pair How big is the difference in signal? What is the principle of application?

A: The use of blind holes or buried holes is an effective way to increase the density of the multilayer board, reduce the number of layers and the size of the board, and greatly reduce the plating pass. The number of holes. However, in comparison, the through hole is well realized in the process and the cost is low, so the through hole is used in the general design.

Q:When designing a PCB, how to consider electromagnetic compatibility EMC/EMI, what needs to be considered? What measures are taken?

A: A good EMI/EMC design must take into account the location of the device, the layout of the PCB stack, and the important way to go online. Device selection, etc. For example, the location of the clock generator should not be close to the external connector. The high-speed signal should go as far as possible to the inner layer and pay attention to the characteristic impedance matching and the continuity of the reference layer to reduce the reflection. The slope of the pushed signal (slew rate) is as small as possible to reduce the high frequency components. When choosing a decoupling/bypass capacitor, pay attention to whether the frequency response meets the demand. Reduce power plane noise. In addition, pay attention to the return path of the high-frequency signal current so that the loop area is as small as possible (that is, the loop impedance is as small as possible) to reduce the radiation. It is also possible to divide the formation to control the range of high-frequency noise. Finally,Appropriately select the chassis ground of the PCB and the enclosure.

Q: How to deal with the avoidance of vias in the high-speed PCB during the wiring process, Any good suggestions?

A: High-speed PCB, it is best to make fewer holes, and increase the signal layer to solve the need to increase the via.

Q: In a circuit that has both analog and digital circuits, PCB How to avoid clocking how to avoid mutual interference?

A: The analog circuit is generally interfered if it matches a reasonable amount of radiation. Sources of interference come from devices, power supplies, space, and PCBs;Digital circuits are definitely sources of interference because of the large number of frequency components. The solution is generally reasonable device layout, power supply decoupling, PCB delamination, and if the interference characteristics are large or the analog part is very sensitive, a shield can be considered.

Q: How do you determine how many layers are needed for PCB design?

A: Consider the line density, BGA and signal 3:

1. Trace density

In general, the layout of the PCB first, and then the number of layers of the evaluation board; after the layout is complete, you can see The signal flow to the board, the traces are not smooth (considering the cross line), and several trace layers are required. The assessment focuses on the most dense areas of the flying line, because the lines in the most dense areas are straightened out, and other loose areas are easy;

2. BGA:

When there is a BGA on the PCB, the assessment focuses on the BGA Depth (the number of pads to the middle pad (usually the pad of the power supply or ground). When the pitch of the BGA pad is above 0.65mm, the pad of the two depths takes a signal line;

3. Signal considerations:

Based on signal quality considerations, it is necessary to add strata (shield cages) ), perform electromagnetic interference shielding and increase the return path. For example, in general, you can use toplayer-signal1-signal2-bottom, but in order to get a better and more stable signal, you can design it as toplayer-GND-signal1-signal2-PWM-bottom (but generally it won’t be because of cost reasons). ).